Method of purification of silicon compounds



tes Patent 2,877,097, Patented Mar. 10, 1959 ice METHOD OF PURIFICATIONOF SILICON COMPOUNDS Guenter A. Wolff, Little Silver, N. I.

No Drawing. Application May 6, 1958 Serial No. 733,451

Claims. (Cl. 23-205) (Granted under Title 35, U. S. Code (1952), sec.266) The invention described herein may be manufactured and used by orfor the Government for governmental purposes without the payment of anyroyalty thereon.

This application is a continuation-in-part of copending applicationSerial No. 595,802, filed July 3, 1956, now Patent No. 2,857,249.

This invention relates to a method of purifying silicon tetrachloride oftraces of boron trichloride.

The conventional methods of purification of silicon compounds haveproved to be successful for all impurities but boron. The reason forthis failure is that according to the diagonal relationship of theperiodic table both elements boron (B) and silicon (Si) or correspondingcompounds such as their halides, e. g., chlorides (BCl SiCl bromides orfluorides dilfer only little in properties.

In the above-identified patent application there is described a methodof purifying silicon tetrachloride (SiCl of traces of boron trichloride(BCl by adding to the silicon tetrachloride a nucleophilic amine of thestructural formula NR wherein R is an alkyl radical such as a methyl orethyl group or a phenyl group. The nucleophilic tertiary amine reactswith the boron trichloride to form the molecular compound CI BNR ofrelatively low solubility or low vapor pressure with respect to theboron trichloride. Therefore, the resulting mixture can be separated byany conventional fractional distillation method into a volatileboron-free fraction and a boroncontaminated residue.

It now has been found that purification of silicon tetrachloride oftraces of boron trichloride can be accomplished in a very simple andvery efficient manner by using instead of a nucleophilic tertiary amineitself an anion exchange resin having attached to its hydrocarbonnetwork nucleophilic tertiary amine groups (-NR or nucleophilicquaternary ammonium groups (NR Cl). The substituents R may be differentin compounds or groups.

An ion exchange resin may be broadly defined as an elastic B-dimensionalhydrocarbon network or polymer chain to which is attached a large numberof ionizable groups. For the purposes of the present invention thehydrocarbon networks or polymer chains of the vinyl type, particuarlythose formed by the copolymerization of styrene and divinyl benzene aresuited best provided they are insoluble in the liquid silicontetrachloride. The following commercially available anion exchangeresins have been particularly eliective in carrying out the processes ofthe present invention:

Dowex 3 made by The Dow Chemical Company and described in more detail inPrinciples of Ion Exchange Processes, publication No. 1, 1954, of theabovementioned company.

Amberlite IRA-400 (C1) made by Rohm & Haas Company, Philadelphia,Pennsylvania, and constituting a strong basic anion exchange resin whichderives its activity from quaternary ammonium groups. It is supplied asthe chloride salt and is also available in chromatographic grade (ahighly purified analytic grade resin).

"Amberlite IRA-401 (C1) is similar to the abovementioned resin but ofhigher porosity.

The above-mentioned resins or similar anion exchange resins that areinsoluble in silicon tetrachloride are used according to the inventionfor the purpose of purifying silicon tetrachloride of traces of borontrichloride by passing the contaminated silicon compound thru one ormore columns containing one or more of the abovementioned anion exchangeresins in the manner illustrated in the following example:

Example.-A glass column of about 2 meter lengths and 3 centimeters indiameter is filled with an anion exchange resin of the typecharacterized above, for instance, a vinyl type resin having attached toits polymer chain a dialkylamine group such as the dimethylamine groupN(CH The liquid silicon tetrachloride to be purified of traces of boronchloride is passed thru the column at a slow rate. To attain completepurification a battery of such columns is preferably used.

Instead of passing the liquid silicon tetrachloride thru the column itis also possible to pass the silicon tetrachloride in gaseous form thrua battery of columns.

The spent nucleophilic anion exchange resin may of course be reactivatedin the conventional manner by flushing it with organic solvents such asbenzene, xylene, etc. To facilitate regeneration it is advantageous toadd to these organic solutions tertiary amines such as trimethylamine orquaternary ammonium compounds which attach themselves to the polymerchain or hydrocarbon network of the original anion exchange resin.

The purified silicon tetrachloride may be transformed by the usualmethods into silicon of transistor grade purity and is particularlyvaluable for applications where complete absence of boron is required.

It will be understood that the above-described processes are capable ofmany modifications and should not be limited to the specific examplesgiven merely for the sake of illustrating the inventive idea as definedin the following claims.

What is claimed is:

1. Method of purifying silicon tetrachloride of traces of borontrichloride comprising passing the silicon tetrachloride to be purifiedthru absorption columns containing an anion exchange resin havingattached to its polymer chain a radical selected from the groupconsisting of tertiary amine radicals (NR2) and quaternary ammoniumradicals (NR Cl) wherein R constitutes a member selected from the groupconsisting of alkyl and phenyl radicals. I 1 mg? 2. Method of purifyingsilicon tetrachloride according to claim 1 in which the anion exchangeresin consists essentially of a vinyl type resin having attached to itshydrocarbon network a nucleophilic tertiary amine of exchange character.

3. Method of purifying silicon tetrachloride according to claim 1 inwhich the anion exchange resin consists essentially of a vinyl typeresin having attached to its hydrocarbon network a quaternary ammoniumcompound of exchange character.

4. Method of purifying silicon tetrachloride according to claim 1 inwhich the silicon tetrachloride is passed in liquid form thru columnsfilled with anion exchange resins.

5. Method of purifying silicon tetrachloride according to claim 1 inwhich the silicon tetrachloride is passed inthe gaseous state thrucolumns filled with anion exchange resins.

References Cited in the file of this patent UNITED STATES PATENTS2,167,358 Gleason July 25, 1939 2,400,874 Burk May 28, 1946 2,400,875Hughes May 28, 1946

1. METHOD OF PURIFYING SILICON TETRACHLORIDE OF TRACES OF BORONTRICHLORIDE COMPRISING PASSING THE SILICON TETRACHLORIDE TO BE PURIFIEDTHRU ABSORPTION COLUMNS CONTAINING AN ANION EXCHANGE RESIN HAVINGATTACHED TO ITS POLYMER CHAIN A RADICAAL SELECTED FROM THE GROUPCONSISTING OF TERTIARY AMINE RADICALS (-NR2) AND QUATERNARY AMMONIUMRADICALS (-NR3CI) WHEREIN R CONSTITUTES A MEMBER SELECTED FROM THE GROUPCONSISTING OF ALKYL AND PHENYL RADICALS.